C-reactive protein (abbreviated as CRP) is one kind of acute phase proteins, and although its blood level rapidly increases on inflammatory diseases or the like accompanying the disorganization, it is present in healthy human blood only in a trace amount (at most 1 .mu.g/ml). Therefore, it is widely used for the diagnosis of purulent diseases, rheumatosis and the like. As shown in FIG. 1, the CRP is a pentamer comprising five disk-like subunits (molecular weight about 20,000), and two monoclonal antibodies to the CRP have been constructed (The Journal of Immunology, Vol. 131, 2411-2415m). These are a monoclonal antibody which specifically reacts with the circular upper face of a disk-like subunit (A-face in FIG. 1; sometimes simply designated "A-face"), but does not react with the circular lower face of the disk-like subunit (B-face in FIG. 1; sometimes simply designated "B-face"); and a monoclonal antibody which reacts with the circular lower face (B-face) of a disk-like subunit, but does not react with the circular upper face (A-face) of the disk-like subunit. Nevertheless, a binding of the former monoclonal antibody or the latter monoclonal antibody to an insoluble carrier, followed by mixing with a CRP-containing sample, does not induce agglutination. Therefore, when the CRP is to be quantitatively measured using the above-mentioned known monoclonal antibodies, it is necessary to use a sandwich assay or the like utilizing both monoclonal antibodies.